1. Field of the Invention
This invention relates generally to tension measuring apparatus and methods, and more particularly to an apparatus and method for measuring tension in guy wires.
2. Background Art
Tall towers, such as radio and television broadcasting towers, microwave towers, etc., may be erected as self-supporting or guyed. The guyed towers are supported by cables running from the tower to the ground called “guy” wires. These guy wires are typically disposed on all three sides of a tower at 120° and hold the tower structure in a vertical position. They also absorb the forces of winds, ice deposits, etc., on the tower. When a tower is designed it is designed as a unit, including the guy wires and the tension in them. The tension in the guy wires can change over time due to the guy elongation created by its own stabilization or equilibrium, shifting ground conditions, damage and/or various temperature and severe weather conditions. It is important that the tension in the wires be kept close to the design value, since a tension greater or lower than designed could allow the tower to bend and perhaps fail under high winds. Uneven tension in the guy wires can distort the tower and make it susceptible to wind damage. Uneven tension can also allow the tower to turn such that the microwave antenna installed on its structure becomes misaligned from the proper alignment with the adjacent microwave antenna from the next tower.
There are two basic methods for measuring guy wire tension in the field; the direct method, and the indirect method. There are two common techniques for the indirect method of measuring guy wire tension; the pulse or swing (vibration) technique, and the tangent intercept or sag technique.
In the direct method, a dynamometer (load cell) with a length adjustment device, such as a come-along, is attached to the guy wire just above the turnbuckle and onto the anchor shaft below the turnbuckle, thus making the turnbuckle redundant. The come-along is then tightened until the entire load is taken off of the turnbuckle. At this point, the dynamometer (load cell) carries the entire guy load to the anchor, and the guy tension may be read directly off of the dynamometer dial. To set the correct tension, the come-along is adjusted until the proper tension is read on the dynamometer. Two control points are marked, one above the clamping point on the guy and one on the anchor shaft, and the control length is measured. The dynamometer and the come-along are removed, and the original turnbuckle is adjusted to maintain the control length previously measured. This method is generally considered to be the most accurate.
The indirect measurement method using the pulse or swing (vibration) technique assumes that the guy wire behaves like a string (its period of vibration changes with a change in tension). The pulse or swing technique involves imparting a pulse to the wire and then measuring the time it takes for the pulse to travel several times up and down the wire. The swing technique involves pushing and pulling perpendicular to the wire to set it swaying like a pendulum. The average time of travel per oscillation, or per swing, can then be converted to tension in the wire using an equation that factors in: the average guy wire tension, mass of the cable, chord length, number of complete vibrations or swings, and time in seconds measured while counting the vibrations or swings.
The indirect measurement method using the tangent intercept or sag technique assumes that the guy wire is suspended as a parabola and that its weight is uniformly distributed along the chord. A line of sight is established that is tangential to the guy wire near the anchor end that intersects the tower leg a distance below the guy wire attachment on the mast. This tangent intercept distance is either measured or estimated, and the tension in the wire using an equation that factors in: the horizontal distance between the tower and anchor, the vertical distance between the anchor and the attachment on the mast, the length of the chord between the anchor and the attachment on the mast, the angle of the line of sight, and the vertical distance between the attachment and where the line of sight intersects the leg of the tower (tangent intercept distance).
When guy wire tension table are not readily available, it may be necessary to calculate the change in guy wire tensions due to the difference between the ambient temperature at the time of measurement and the assumed ambient temperature for setting the initial tension. This is typically carried out using a complicated mathematic formula, or a computer program.
Published standards for steel antenna towers and antenna-supporting structures state that for new structures, the guy wire tensions should be set at +15% and −5% of the specified initial tension at anchorage, corrected for the ambient temperature. For existing structures, where constructional stretch of the cables has occurred, the tolerance should be +/−10%. Measurements shall be made at a time when the wind velocity is less than 25 km/h at ground level and there is no ice on the cables.
There are several patents directed toward apparatus for measuring tension in guy wires. Davies, U.S. Pat. No. 2,376,037 discloses an apparatus for measuring and adjusting tension in guy lines that are connected to an embedded anchor bar through a series of links and a turnbuckle. The device has a first yoke in the form of a pair of parallel plates that is detachably secured to the upper end of the cable socket above the turnbuckle and a similar second yoke that is detachably secured to a lowermost one of the links by a pin extending through a second hole provided in the link. Alternatively, the first yoke is detachably secured to the lower eye bolt of the turnbuckle by a pin connecting the turnbuckle to the uppermost link. The outer ends at one side of the yokes are connected by a threaded rod with yoke-engaging nuts or by a turnbuckle, and the opposite outer ends are connected by a pair of threaded rods or a pair of shackles having a dynamometer interposed therebetween. The nuts are tightened to decrease the distance between the yokes such that the links are relieved of tension and allowed to slacken, and the tension is transferred to the yokes and indicated by the dynamometer. Shortening of the guy line is accomplished by adjusting the guy line turnbuckle and/or by replacing one of the links with a shorter link.
Grade et al, U.S. Pat. No. 4,423,639 discloses an apparatus for adjusting and indicating the tension in a guy line system of an inclined oilwell derrick. The tension indicator includes a hydraulic compression load cell, a pair of interlocking U-shaped yokes housing the load cell between the bases of the U-shaped portions, a plunger connected to the load cell bearing against the base of one interlocking yoke, the load cell bearing against the base of the other interlocking yoke, a walking load binder mounted on the outer end of a first interlocking yoke and providing an initial tension load to the guy line system, a threaded nut and screw adjustment assembly affixed to the outer end of a second interlocking yoke and providing a finer adjustment of the tension provided by the walking load binder, a pair of attachment hooks rotatably mounted on the outer ends of the tension indicator device for inserting the tension indicator device between a set of intermediate chains connected in a guy line, and a calibrated pressure gauge directly attached to the load cell. The interlocking yokes transfer a linear tension load in the guy line into a compressive in the load cell and the compressive force is transmitted as a pressure signal to the pressure gauge.
Burbank, Jr., U.S. Pat. No. 4,534,228 discloses a wireline tension gage for mounting upon a wireline between its load and free ends. The tension gage has a body with a central post to engage the wireline in a saddle mount and connected with a transmitter which produces a pneumatic signal indicative of wireline forces acting on the post. Rollers at each end of the body engage and laterally displace at certain angles the wireline to each side of the post. The rollers are mounted on eccentric pivots whereby the rollers can be moved to release or engage the wireline over predetermined distances. Swinging bridge roller mounts allow the body to be placed with the post against the wireline, and then, locked to it in operative position. An adjustment calibrates the transmitter to zero line tension conditions for using the tension gage as an accurate weight indicator.
There are also several well know commercially available devices commonly called “tensionmeters” that are used for measuring tension in a cable. For example, Penn-Tech International, of West Chester, Pa., markets a cable tensionmeter instrument that is suspended on the cable with hooks and measures cable tension by reading deflection of the cable. There are 11 different models for cables with diameters between 3/16″ and 1″, and calibration charts are plotted specifically for each unit.
Checkline Europe, of Enschede, Netherlands, markets a digital cable tensionmeter instrument that is placed on the cable under a predefined angle. The force used to generate the angle is then used to determine the tension in the cable. The gauge processor of the tensionmeter is calibrated for each cable that is measured and directly displays the tension value. The reference calibrations values are memorized in the processor memory per cable model.
The present invention is distinguished over the prior art in general, and these patents in particular by an apparatus for measuring tension in guy wires, line or cable systems wherein the lower end of the guy wire has a thimble adjoined by a connection pin or clevis pin to a yoke or clevis of a turnbuckle shackle bolt connected to an earth anchor. The apparatus has a generally rectangular lower block assembly that is removably received beneath the yoke or clevis and a generally rectangular upper jaw assembly that is removably received on the guy wire above the clevis, which has an internal thimble-engaging member. One end of the lower block assembly and upper jaw assembly are connected by an elongate threaded rod, and their opposed end is connected by a threaded rod assembly that includes a tension sensor connected with an indicator device. Nuts installed on the lower ends of the threaded rod and threaded rod assembly beneath the lower block assembly are rotated to draw the jaw assembly downward to engage the internal thimble-engaging member on the upper ends of the thimble and move it downward such that the tension on the clevis pin is relieved and the tension is transferred to the threaded rod and threaded rod assembly and sensed by the tension sensor and indicated by the indicator device.